Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations, Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems, such as the Freehand system by NeuroControl (Cleveland, Ohio), have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
These implantable neurostimulation systems typically include one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site, and a neurostimulator (e.g., an implantable pulse generator (IPG)) implanted remotely from the stimulation site, but coupled either directly to the neurostimulation lead(s) or indirectly to the neurostimulation lead(s) via a lead extension. The neurostimulation system may further comprise an external control device to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters.
Electrical stimulation energy may be delivered from the neurostimulator to the electrodes in the form of an electrical pulsed waveform. Thus, stimulation energy may be controllably delivered to the electrodes to stimulate neural tissue. The combination of electrodes used to deliver electrical pulses to the targeted tissue constitutes an electrode combination, with the electrodes capable of being selectively programmed to act as anodes (positive), cathodes (negative), or left off (zero). In other words, an electrode combination represents the polarity being positive, negative, or zero. Other parameters that may be controlled or varied include the amplitude, width, and rate of the electrical pulses provided through the electrode array. Each electrode combination, along with the electrical pulse parameters, can be referred to as a “stimulation parameter set.”
With some neurostimulation systems, and in particular, those with independently controlled current or voltage sources, the distribution of the current to the electrodes (including the case of the neurostimulator, which may act as an electrode) may be varied such that the current is supplied via numerous different electrode configurations. In different configurations, the electrodes may provide current or voltage in different relative percentages of positive and negative current or voltage to create different electrical current distributions (i.e., fractionalized electrode combinations).
As briefly discussed above, an external control device can be used to instruct the neurostimulator to generate electrical stimulation pulses in accordance with the selected stimulation parameters. Typically, the stimulation parameters programmed into the neurostimulator can be adjusted by manipulating controls on the external control device to modify the electrical stimulation provided by the neurostimulator system to the patient. Thus, in accordance with the stimulation parameters programmed by the external control device, electrical pulses can be delivered from the neurostimulator to the stimulation electrode(s) to stimulate or activate a volume of tissue in accordance with a set of stimulation parameters and provide the desired efficacious therapy to the patient. The best stimulus parameter set will typically be one that delivers stimulation energy to the volume of tissue that must be stimulated in order to provide the therapeutic benefit (e.g., treatment of pain), while minimizing the volume of non-target tissue that is stimulated.
However, the number of electrodes available, combined with the ability to generate a variety of complex stimulation pulses, presents a huge selection of stimulation parameter sets to the clinician or patient. For example, if the neurostimulation system to be programmed has an array of sixteen electrodes, millions of stimulation parameter sets may be available for programming into the neurostimulation system. Today, neurostimulation system may have up to thirty-two electrodes, thereby exponentially increasing the number of stimulation parameters sets available for programming.
To facilitate such selection, the clinician generally programs the neurostimulator through a computerized programming system. This programming system can be a self-contained hardware/software system, or can be defined predominantly by software running on a standard personal computer (PC). The PC or custom hardware may actively control the characteristics of the electrical stimulation generated by the neurostimulator to allow the optimum stimulation parameters to be determined based on patient feedback or other means and to subsequently program the neurostimulator with the optimum stimulation parameter set or sets, which will typically be those that stimulate all of the target tissue in order to provide the therapeutic benefit, yet minimizes the volume of non-target tissue that is stimulated. The computerized programming system may be operated by a clinician attending the patient in several scenarios.
For example, in order to achieve an effective result from SCS, the lead or leads must be placed in a location, such that the electrical stimulation will cause paresthesia. The paresthesia induced by the stimulation and perceived by the patient should be located in approximately the same place in the patient's body as the pain that is the target of treatment. If a lead is not correctly positioned, it is possible that the patient will receive little or no benefit from an implanted SCS system. Thus, correct lead placement can mean the difference between effective and ineffective pain therapy. When electrical leads are implanted within the patient, the computerized programming system, in the context of an operating room (OR) mapping procedure, may be used to instruct the neurostimulator to apply electrical stimulation to test placement of the leads and/or electrodes, thereby assuring that the leads and/or electrodes are implanted in effective locations within the patient.
Once the leads are correctly positioned, a fitting procedure, which may be referred to as a navigation session, may be performed using the computerized programming system to program the external control device, and if applicable the neurostimulator, with a set of stimulation parameters that best addresses the painful site. Thus, the navigation session may be used to pinpoint the volume of activation (VOA) or areas correlating to the pain. Such programming ability is particularly advantageous for targeting the tissue during implantation, or after implantation should the leads gradually or unexpectedly move that would otherwise relocate the stimulation energy away from the target site. By reprogramming the neurostimulator (typically by independently varying the stimulation energy on the electrodes), the volume of activation (VOA) can often be moved back to the effective pain site without having to re-operate on the patient in order to reposition the lead and its electrode array. When adjusting the volume of activation (VOA) relative to the tissue, it is desirable to make small changes in the proportions of current, so that changes in the spatial recruitment of nerve fibers will be perceived by the patient as being smooth and continuous and to have incremental targeting capability.
One known computerized programming system for SCS is called the Bionic Navigator®, available from Boston Scientific Neuromodulation Corporation. The Bionic Navigator® is a software package that operates on a suitable PC and allows clinicians to program stimulation parameters into an external handheld programmer (referred to as a remote control). Each set of stimulation parameters, including fractionalized current distribution to the electrodes (as percentage cathodic current, percentage anodic current, or off), may be stored in both the Bionic Navigator® and the remote control and combined into a stimulation program that can then be used to stimulate multiple regions within the patient.
Prior to creating the stimulation programs, the Bionic Navigator® may be operated by a clinician in a “manual mode” to manually select the percentage cathodic current and percentage anodic current flowing through the electrodes, or may be operated by the clinician in an “automated mode” to electrically “steer” the current along the implanted leads in real-time (e.g., using a joystick or joystick-like controls), thereby allowing the clinician to determine the most efficacious stimulation parameter sets that can then be stored and eventually combined into stimulation programs. In the context of SCS, current steering is typically either performed in a rostro-caudal direction (i.e., along the axis of the spinal cord) or a medial-lateral direction (i.e., perpendicular to the axis of the spinal cord). The Bionic Navigator® may use one of two ways to electrically steer the current along the implanted leads.
In one current steering method, known as “weaving,” the anode or anodes are moved around the cathode, while the cathode slowly progresses down the sequence of electrodes. In the context of SCS, the active electrode combinations typically used to implement the weaving sequence includes a narrow tripole (tightly spaced center cathode and two flanking anodes), narrow upper bipole (tightly spaced anode above cathode), wide upper bipole (widely spaced anode above cathode), wide tripole (widely spaced center cathode and two flanking anodes), wide lower bipole (widely spaced anode below cathode), narrow lower bipole (narrowly spaced anode below cathode). In another current steering method, known as “panning,” a pre-defined electrode combination is shifted down the sequence of electrodes without changing the basic form of the electrode combination. These current steering methods may have different clinical uses (e.g., finding the “sweet spot” in the case of panning, or shaping the electrical field around the cathode in the case of weaving).
In the context of SCS, a volume of activation (VOA) will typically be displaced in concordance with the displacement of the cathode or group of cathodes as electrical current is steering in a particular direction. In one method, the VOA may be rostro-caudally displaced along the spinal cord of the patient using, e.g., the weaving or panning steering current steering methods, in order to stimulate the rostro-caudal dermatome associated with the ailment to be treated. In another method known as Transverse Tripole Stimulation (TTS), wherein a tripole electrode configuration consisting of a central cathode and two flanking anodes is used, to selectively stimulate dorsal column (DC) nerve fibers without stimulating the dorsal root (DR) nerve fibers typically associated with painful or otherwise uncomfortable side-effects. To target a population of DC nerve fibers where medio-lateral fiber distribution is mapped into rostro-caudal dermatomes, steering of current onto the DC nerve fibers is a critical component of TTS. Typically, in the three-column electrode arrangement, steering of current in TTS can be achieved by adjusting the intensity of the flanking anodes, as described in U.S. patent application Ser. No. 12/508,407, entitled “System and Method for Increasing Relative Intensity Between Cathodes and Anodes of Neurostimulation System,” which is expressly incorporated herein by reference.
The Bionic Navigator® presently performs current steering in accordance with a steering or navigation table. For example, an exemplary navigation table, which includes a series of reference electrode combinations (e.g., for a lead of 8 electrodes) with associated fractionalized current values (i.e., fractionalized electrode configurations), can be used to gradually steer electrical current from one basic electrode combination to the next, thereby electronically steering the volume of activation (VOA) along the leads.
While the use of navigation tables have proven to be useful in steering electrical current between electrodes in an efficient manner, that are certain inherent disadvantages associated with navigation tables. For example, assuming a current step size of 5% in the navigation table, there are literally billions of fractionalized electrode configurations that can be selected. However, due to memory and time constraints, only a limited number of fractionalized electrode configurations are stored within the navigation table. Therefore, not every desired electrode combination and associated fractionalized current values can be represented within a steering table.
Furthermore, a substantial amount of time and effort must be spent in developing navigation tables for each new lead design, thereby presenting a bottleneck for lead development. For example, each steering table must take into account the variability in electrode position or stimulation input. The variability in electrode position may be due to, e.g., a different lead model, different lead configurations (e.g., a closely spaced side-by-side configuration, a closely spaced top-bottom configuration, a widely spaced top-bottom configuration, or a widely spaced side-by-side configuration), stagger of the leads, etc. The variability in stimulation input may be due to, e.g., the development or inclusion of additional steerable fields (e.g., medio-lateral tripole steering), upgrades in steering controls (e.g., focusing/blurring of fields, anode intensification or de-intensification (i.e., increasing or decreasing local anodic current relative to cathodic current), current steering from different screens, etc. Because the implementation of new navigation tables must take into account all leads that are to be used with the IPG, as well as the different lead positions, this challenge slows the ability to include new navigation features in the system.
Furthermore, if the remote control needs to be reprogrammed; for example, if the patient returns to a physician's office to be refitted to improve the stimulation therapy provided by the neurostimulator, the clinician may have to start the fitting from scratch. In particular, while the remote control is capable of uploading the stimulation parameter sets to the Bionic Navigator® to aid in reprogramming the remote control, they may be different from any stimulation parameter sets that are capable of being generated using the navigation table due to the limited number of fractionalized electrode configurations within the navigation table; that is, the fractionalized electrode configurations currently stored in the remote control may not match any fractionalized electrode configurations stored in the navigation table because they were originally generated when the Bionic Navigator® was operated in the manual mode.
In any event, if the stimulation parameter sets uploaded from the remote control to the Bionic Navigator® do not identically match any stimulation parameter set corresponding to a fractionalized electrode configuration stored in the navigation table, it cannot be used as a starting point in reprogramming the remote control/IPG. As a result, the amount of time required to reprogram the remote control/IPG may be as long as the amount of time required to originally program the remote control/IPG with the Bionic Navigator®. Because programming the remote control can be quite complex, even when the Bionic Navigator® is operated in the navigation mode, the time lost as a result of having to reprogram the remote control/IPG from scratch, can be quite significant.
In one novel method, described in U.S. Pat. No. 8,412,345, which is incorporated herein by reference, a stimulation target in the form of an ideal target pole (e.g., an ideal bipole or tripole) is defined and the stimulation parameters, including the fractionalized current values on each of the electrodes, are computationally determined in a manner that emulates these ideal target poles. It can be appreciated that current steering can be implemented by moving the ideal target poles about the leads, such that the appropriate fractionalized current values for the electrodes are computed for each of the various positions of the ideal target pole. As a result, the current steering can be implemented using an arbitrary number and arrangement of electrodes, thereby solving the afore-described problems.
While the computation of stimulation parameters to emulate ideal target poles is quite useful, there remains a need to provide a more generalized format for ideal target poles to provide more flexibility to steering current in an arbitrary direction. For example, ideal target poles aligned along the longitudinal axis of the spinal cord of a patient may be optimum when steering current in a rostro-caudal (longitudinal) direction, but may not be optimum when steering current in a medial-lateral (transverse) direction. Likewise, ideal target poles aligned perpendicular to the longitudinal axis of the spinal cord of a patient may be optimum when steering current in a medial-lateral (transverse) direction, but may not be optimum when steering current in a rostro-caudal direction (longitudinal). There also remains a need for improved techniques using ideal target poles to steer current in the rostro-caudal direction and the medial-lateral direction.
Furthermore, because there a limited number of electrodes when steering current in a particular direction using arbitrarily defined target poles, there remains a need to modify the current steering on-the-fly to prevent any target poles from being moved outside the maximum extent of the electrode array. Also, for ideal multipole configurations, which include at least one ideal cathode and at least one ideal anode, it is desirable to match the spacing between the ideal cathode(s) and ideal anode(s) with the spacing of the physical electrodes in order to minimize dilution of the electrical current on multiple electrodes, which may otherwise cause amplitude fluctuation or a non-focused stimulation region during current steering. However, because different types of neurostimulation leads have different electrode separations, a current steering algorithm that is designed for a particular electrode separation cannot be used for other electrode separations. Furthermore, when multiple neurostimulation leads are used, the spacings between the electrodes will typically not be uniform, thereby providing a challenge when attempting to match the ideal cathode/anode spacings with the spacings of the physical electrodes.